hi tilo,

you probably solved your problem in the meantime. nevertheless,
because i encountered the same problem today, and for the potential
benefit of others, i post this late reply.

> The following providers:
> oracle.toplink.essentials.PersistenceProvider
> oracle.toplink.essentials.ejb.cmp3.EntityManagerFactoryProvider
> Returned null to createEntityManagerFactory.
>
> That is why I am still convinced that the problem is that I was not
> able to switch completely from Toplink to Hibernate just by changing
> the persistence provider in persistence.xml.

in addition to fixing persistence.xml, you also need to remove the
toplink libraries from the class path (cf. libraries node of the
project tree), and replace them with the ones provided by openJPA,
namely geronimo-jpa_xxx.jar which provides the appropriate
javax.persistence.* classes.

--
matthias